# Tag Info

Essentially, if there is no arbitrage, then for any ratio of prices of assets $X_t/N_t =: M_t$, there must be a measure (depending on $N$ only) such that the ratio is a martingale (under that measure): $E_N[ X_T/N_T \,| F_t ] = X_t/N_t$. ...which is of course what you wrote, I'm just highlighting that it holds for any two assets. An asset here is ...